Trace metal concentrations in the ocean have recently become of concern because these may be indicative of pollution levels and hence, have an environmental impact. In inshore waters possibly harmful concentrations of trace metals are more common. The sacrificial zinc and lead anodes attached to ships release these metals into the surrounding waters. In some areas, shore installations discharge solutions having cadmium and lead ions into the coastal waters. Anti-fouling paints constantly leach copper which may pose an environmental hazard in poorly flushed bays or shipping lanes having substantial traffic. Thus, in each of these instances, not only is it important to be able to accurately measure the metal concentrations, but it also behooves scientists to be able to understand which chemical forms of these metals are present since the latter determines their toxicity and ultimate disposition of the metals. For example, although copper, zinc, cadmium, and lead are generally released as ions (the toxic form), there are indications that these metals form colloidal hydroxides in the seawater and are removed from solution by agglomeration or precipitation or by absorption or other particulate matter.
Of even more general oceanographic interest is the effect of trace metals on marine primary productivity. It is generally believed that the ionic form of copper even at the low concentrations found in the oceans inhibits the phytoplankton production and that the copper must be first chelated by planktonic exudates before full growth can occur. It is believed that this might explain the low productivity of newly upwelled waters. An investigation therefore, of trace metal concentrations in these areas also is of interest to either validate or disprove a multitude of theories.
The need therefore, is apparent for a technique or system for performing in-situ trace metal analyses which reliably indicates the presence and concentration of metals of interest. One such apparatus has been brought to the state of practical application and is described in U.S. Pat. No. 3,904,487 to Stephen H. Liberman and Alberto Zirino, one of the present coinventors, and entitled "Anodic Stripping Voltammetry and Apparatus Therefor". This apparatus pumps a mercury plating solution through a tubular mercury graphite electrode functioning as a working electrode and a tubular reference electrode. A thin film of mercury is deposited on the inner surface of the working electrode when a plating potential is applied. Next, a seawater sample is pumped through the electrodes and trace metals are reduced onto the mercury film when the plating potential is reconnected. After a predetermined time, a pulsed scanning potential gradient is applied across the two electrodes. At discrete levels within the pulsed scanning potential gradient certain ones of the trace metals are stripped from the active metal film and the magnitudes of the currents at these levels are monitored and recorded. The magnitudes of the currents at the discrete potential levels are proportional to the concentrations of the trace metals, zinc, cadium, lead or copper. The system is reactuated to reintroduce the flowing mercury plating solution through the working electrode to deposit another thin film of mercury. Trace metals are redeposited and stripped from the electrode for successive analyses. Another not so apparent limitation arose by using a reference electrode made of a silver chloride covered silver wire. The silver chloride itself would be stripped into the seawater sample affecting the validity of an analysis. The silver ion tended to obscure the actual concentrations of certain metals of interest. The erosion of the silver ions from preceeding samples affected the subsequent readings. It was found that the silver ion from the reference electrode particularly influenced the copper ion determination to create stability problems for repetitive analyses of several seawater samples. The stability otherwise inherent in a silver electrode was compromised by the current flow through it and the resultant stripping away of the silver ion into the samples. Therefore, there is a continuing need in the state-of-the-art for a more stable reference electrode which helps provide more accurate representations of trace metal concentrations in seawater.